The present invention is directed to a rock drill with an axially extending drill shaft having at least one helical drilled material removal groove extending in its axial direction. A drill head formed of a hard metal alloy is securely attached to one end of the drill shaft. The drill head has a pair of intersecting diagonals with a main cutter extending along one of the diagonals and at least one supplementary cutter extending along the other diagonal.
Rock drills are used in axially directed impact-supported rotary drilling equipment normally used for forming boreholes or openings in concrete or masonry by rotating impacts. It is well known, when drilling in hard rock and at high drilling speeds, rock drills wear very rapidly in the region of the drill tip. Accordingly, rock drills are faced with a hard metal alloy. In the present case, hard metal alloys are defined as centered or fused carbide, silicides, borides or their alloys.
Such a rock drill is disclosed in EP-A-O 654 580. This rock drill has a drill head formed, as a whole, of a hard metal connected with the drill shaft by soldering or welding. The cross section of the drill head is essentially rectangular, with cutters arranged on each diagonal. The cutters serve for wearing away the rock and provide lateral guidance for the rock drill in the borehole. The cutters are composed of a main cutter extending along one diagonal, and two supplementary cutters arranged parallel to another diagonal, inclined with respect to the first diagonal. The cutters are positioned in such a way that the main cutter and the supplementary cutters describe the same enveloping curve as the rock drill is rotated. The main cutter is roof-shaped and centers the rock drill while assuring the main removal of the drilled material, in that it engages the rock being worked before the supplementary cutters do. The drilled material or drillings, removed by the main cutter, are thrown into the radially outer peripheral region of the drillhead and are further comminuted by the supplementary cutters, which are set back axially with respect to the main cutter.
This known rock drill is distinguished by a high drilling performance, a long service of life and good discharge of the drilled material. Nevertheless, there is the desire to improve the drilling performance even further. In particular, the rock drill is to be further developed for optimizing the cutters with respect to their specific stress. In this regard, it should be noted, that due to the axial impact of the rotary impact drill, the central part of the main cutter is subjected to stresses different from those experienced by the radially outer regions of the main cutter or the supplementary cutters, such as shearing forces. Especially when a borehole is being drilled, the total impact energy of the rotary impact drill must be absorbed by the central part of the main cutter.